This invention relates to the compound tin mesoporphyrin, to therapeutically useful compositions containing it, and to the use of the compound and the compositions in treating various metabolic afflictions of mammals, particularly humans.
Heme is a red pigment comprised of four subunits called pyrroles; these subunits are chemically joined to form a single large tetrapyrrole (porphyrin) ring structure. A metal atom is chelated at the center of this porphyrin: in higher organisms this metal is iron and the porphyrin ring structure is called protoporphyrin IX. In physiological systems heme is bound to certain proteins; these hemeproteins bind oxygen at the site of the metal atom or they function as components of membrane bound electron transport systems. Cellular respiration, energy generation and chemical oxidations are dependent on these hemeproteins.
In mammals and other vertebrates heme is oxidatively degraded by heme oxygenase to form the open chain tetrapyrrole biliverdin. In mammals biliverdin is reduced to bilirubin by biliverdin reductase. In liver bilirubin is converted to the mono- and di-glucuronide conjugates by the hepatic glucuronyl transferase system prior to its excretion.
Bilirubin is a toxic compound, but normally this toxicity is not manifest since bilirubin is rapidly bound to plasma proteins, transported to liver, conjugated and excreted. However in the newborn high undesirable concentrations of bilirubin exist in serum and may produce neurotoxicity. The intractable neurological syndrome known as "kernicterus" is the most severe manifestation of bilirubin toxicity.
The basis of this neonatal hyperbilirubinemia lies in a number of factors, mainly the rapid hemolysis of fetal erythrocytes after birth and a developmental immaturity of the hepatic conjugating system which normally facilitates the excretion of bilirubin via the bile. The levels of heme oxygenase, the rate limiting enzyme in the catabolism of heme to bilirubin are also markedly elevated at this time resulting in high rates of production of this bile pigment. Current methodologies for suppressing severe neonatal jaundice include a. stimulation of the hepatic conjugating system for bilirubin by drugs, e.g. phenobarbital, b. partial exchange transfusion, and c. phototherapy. None of these methods is fully satisfactory since there are as yet many unanswered questions with respect to their safety. In addition all these methods are directed towards the disposition of bilirubin once it has been formed in the heme degradative sequence--a complex process to undertake at best.
Elevated levels of bilirubin also often appear in the serum of individuals with diseases such as congenital anemias, thalassemia and sickle cell anemias as well as various forms of liver disease. The concentration of bilirubin in the serum of such individuals rarely reaches the high levels found in neonates. It does, however, attain levels which can be toxic and should be controlled.
It is therefore desirable to have available methods and materials to inhibit the catabolism of heme in order to prevent the accumulation of bilirubin in serum.
Copending and commonly assigned U.S. patent application Ser. No. 684,169, now abandoned, describes the use of tin protoporphyrin IX in the treatment of elevated levels of bilirubin in neonates and adults.
Maintenance of a proper equilibrium or balance of tissue heme content is essential to the normal physiological functioning of cells. When this equilibrium is disturbed by any condition characterized by excess heme--as exemplified by the circumstances listed above--it would be clinically valuable to have a pharmacological mechanism for restoring the equilibrium state of heme in cells by facilitating the excretion of the excess amount of heme from the body.
In association with but independent of the conditions described above, excess iron also accumulates in the body and this accumulation of the metal over time can produce deleterious and even lethal consequences for the host. This excess of iron may derive from several sources; e.g. cooking methods (iron pots) or directly via the diet (e.g., iron-overload induced cutaneous porphyrin), from excess therapeutic administration of the metal in an attempt to vigorously treat unresponsive anemias; from hypertransfusions to which certain patients with blood disorders are subject; idiopathically from the disorders collectively known as "hemachromatosis"; from certain industrial exposures; but the most common causes of excess iron deposition in tissues, and the resultant pathologic consequences which derive thereof, are a consequence of common congenital hemolytic anemias such as sickle cell disease, the various forms of thalassemia, G-6-PD deficiency, hereditary spherocytosis and the like. In these disorders, a greatly shortened red cell life span results in continuous large depositions of iron in tissues to an extent exceeding the capacity of the body to re-utilize the metal. Thus tissue concentrations of iron rise to very high, toxic levels and lead to impairment of vital organ functions manifest for example by cardiomyopathy, pancreatic insufficiency (diabetes) and generalized endocrine failure.
There is no physiological mechanism for excreting this excess of iron and the only generally available therapeutic modality for this purpose is a pharmacological agent known as desferrioxamine. This agent is not specific for iron however and chelates other metals as well; it must in order to be reasonably effective be given intramuscularly and causes substantial local inflammation at the site of injection. Further, original suggestions that it was non-toxic have proved incorrect and a large number of toxic reactions in treated patients have now been reported to occur after its use, including hypotension and allergic reactions.
Sn- protoporphyrin (Sn-PP) as described in copending and commonly assigned U.S. patent application Ser. No. 691,459 manifests the extremely advantageous property of greatly enhancing the biliary excretion of iron into the intestinal contents where the metal is eliminated. Sn-PP acts in this additional fashion by blocking the binding of heme to heme oxygenase, thus preventing the release of iron which normally occurs in the process of heme catabolism and allowing one atom of iron to be excreted into the intestine with every molecule of uncatabolized heme.
Tryptophan is an essential amino acid which has profound effects on a number of metabolic pathways in the whole animal, including man, particularly in the nervous system. Tryptophan is metabolized principally in the liver. Tryptophan which is not metabolized in the liver accumulates in the plasma and in the brain. Brain levels of tryptophan are dependent on plasma levels of the amino acid which in turn are regulated by liver tryptophan pyrrolase. Tryptophan in the brain is metabolized by a different route than in the liver. One of the principal metabolic products of tryptophan in the brain is 5-hydroxytryptamine, or serotonin. The concentrations of tryptophan and serotonin in the brain are closely regulated in humans. Increased concentration of these products are associated with hepatic encephalopathy and migraine headaches. Encephalopathy is a known affliction characterized by degenerative changes in the brain cells leading to confused states and other abnormal behaviour patterns as well as convulsions, stupor and coma. Decreased concentrations of these products have been implicated in narcolepsy, depression and myoclonic disorders characterized by uncontrolled jerky movements.
Tryptophan pyrrolase is an enzyme which occurs in the liver of humans. It catalyzes the oxidative cleavage of tryptophan to N-formylkynurenine and is the first and rate-limiting enzyme in the catabolism of tryptophan in the liver. The active holoenzyme is normally about 50% saturated with heme, but fluctuations in the availability of cellular heme produce rapid changes in the enzyme activity by converting the inactive, heme-free apoenzyme to the active heme containing holoenzyme.
More specifically, and as described in copending and commonly assigned U.S. patent application Ser. No. 691,460, an increase in the amount of heme in the liver as can be produced by parenteral administration of Sn-PP as a result of the ability of this compound to block the catabolism of heme causes increased saturation of tryptophan pyrrolase as the active form of the enzyme. The increased activity of the enzyme resulting from its increased saturation with heme causes an increased rate of tryptophan metabolism in the liver. As a result there is less spill-over of intact tryptophan into the plasma and, ultimately, less accumulation of tryptophan and serotin in the brain.